Carbodiimides are compounds containing the group, --N.dbd.C.dbd.N--. Methods utilized in the past to prepare carbodiimides are often time-consuming and of limited applicability. A well known method involves the reaction of organic isocyanates with phosphine-imines to form carbodiimides, i.e., EQU RNCO+R.sub.3 'P.dbd.NR".fwdarw.RN.dbd.C.dbd.NR"+R.sub.3 'PO
the phosphine-imines, however, are somewhat laborious to prepare. In addition to the use of phosphine-imines other methods are available for preparing carbodiimides. These methods include the conversion of thioureas in the presence of mercuric oxide to carbodiimides, and the oxidation of certain dialkyl thioureas with alkaline sodium hypochlorite. Such methods, however, are accompanied by contamination with undesirable side products which may render the resulting polycarbodiimide unsuitable for certain applications.
Another well known method involves conversion of isocyanates to carbodiimides in the presence of certain phospholine oxides, e.g., ##STR1## This method of preparation is detailed in U.S. Pat. Nos. 2,663,737; 2,853,473; 3,755,242 as well as in T. Campbell, J. Monagle and V. Foldi, Conversion of Isocyanates to Carbodiimides with Phospholine Oxide Catalyst, 84 J. Amer. Chem. Soc. 3673 (1962). The mechanism proposed for this catalyzed reaction involves the intermediate formation of a phosphinimide as a result of a nucleophilic attack by the oxygen atom, of a polarized phosphorous-oxygen bond present in the catalyst, on the isocyanate carbonyl with the subsequent reaction of the imide with a second molecule of isocyanate. Accordingly, it has been observed that in general, compounds containing a polar coordinate bond between the phosphorous and oxygen atoms (i.e., P.fwdarw.O known as a phosphoryl group) appear to derive their catalytic activity from the same mechanism and therefore to have similar, although different, degrees of catalytic activity for converting isocyanates to carbodiimides. Such compounds would include, in addition to phosphine oxides, phosphinates, phosphonates, phosphates, phosphonamides, phosphoramides and phospholine oxides. For a detailed discussion of phosphoryl containing carbodiimide catalytic mechanisms see J. Monagle, Conversion of Isocyanates to Carbodiimides Catalytic Studies, 27 J. Org. Chem. 3851 (1962).
The use of phospholine oxides to prepare carbodiimides is accompanied by certain disadvantages such as lack of commercial availability and extended preparation times. In addition, these compounds are extremely reactive as catalysts and consequently must be utilized at low concentrations and in dilute solutions of isocyanates. On the other hand, many of the other phosphoryl containing catalysts are very sluggish and take an inordinate amount of time to catalyze conversion of isocyanates to carbodiimides. The catalysts of the present invention offer a compromise to the above-described disadvantages in that they are readily available and possess moderate catalytic activity which render them extremely suitable for the preparation of carbodiimides and particularly polycarbodiimides.
It is therefore the object of the present invention to provide a new process for the production of monocarbodiimides and polycarbodiimides within a reasonable period of time.
A further object is to provide a new catalyst for the preparation of monocarbodiimides and polycarbodiimides from all types of isocyanates.
It is still a further object of the present invention to provide a process for utilizing a catalyst which is commercially available, easy to prepare, and can be used neat, or in solution.
These and other objects, as well as the scope, nature and utilization of the claimed invention will be apparent to those skilled in the art from the following detailed description and appended claims.